Regenerated cellulose lyocell fiber is well known. Generally, lyocell fiber is made from reconstituted cellulose spun from aqueous amine oxide solution. An exemplary process is to spin lyocell fiber from a solution of cellulose in aqueous tertiary amine N-oxide; for example, N-methylmorpholine N-oxide (NMMO). The solution is typically extruded through a suitable die into an aqueous coagulating bath to produce an assembly of filaments. These fibers have been widely employed in textile applications. Inasmuch as lyocell fiber includes highly crystalline alpha cellulose it has a tendency to fibrillate which is undesirable in most textile applications and is considered a drawback. In this regard, U.S. Pat. No. 6,235,392 and United States Patent Application Publication No. 2001/0028955 to Luo et al. disclose various processes for producing lyocell fiber with a reduced tendency to fibrillate.
On the other hand, fibrillation of cellulose fibers is desired in some applications such as filtration. For example, U.S. Pat. No. 6,042,769 to Gannon et al. discloses a process for making lyocell fibers which readily fibrillate. The fibers so produced may be treated with a disintegrator as noted in Col. 5 of the '769 patent. See lines 30+. See, also, U.S. Pat. No. 5,725,821 of Gannon et al. Highly fibrillated lyocell fibers have been found useful for filter media having a very high degree of efficiency. In this regard, note United States Patent Application No. 2003/0168401 and United States Application Publication No. 2003/0177909 both to Koslow.
It is known in the manufacture of absorbent sheet to use lyocell fibers having fiber diameters and lengths similar to papermaking fibers. In this regard U.S. Pat. No. 6,841,038 to Horenziak et al. discloses a method and apparatus for making absorbent sheet incorporating lyocell fibers. Note FIG. 2 of the '038 patent which discloses a conventional through-air dried process (TAD process) for making absorbent sheet. U.S. Pat. No. 5,935,880 to Wang et al. also discloses non-woven fibrous webs incorporating lyocell fibers. See also, United States Patent Application Publication No. 2006/0019571. Such fibers have a tendency to flocculate and are thus extremely difficult to employ in conventional wet-forming papermaking processes for absorbent webs. Moreover, conventional lyocell fiber is used in the '038 patent, for example, at elevated weight fractions (40% of wire side layer, Example 1) in order to impact sheet properties.
While the use of lyocell fibers in absorbent structures is known, it has not heretofore been appreciated that very fine lyocell fibers or other regenerated cellulose fibers with extremely low coarseness can provide unique combinations of properties such as wet strength, absorbency and softness even when used in papermaking furnish in limited amounts. In accordance with the present invention, it has been found that regenerated cellulose microfiber can be readily incorporated into a papermaking fiber matrix of hardwood and softwood to enhance networking characteristics and provide premium characteristics even when using less than premium papermaking fibers.
It has been disclosed in U.S. Pat. No. 6,461,476 to Goulet et al. that the wet/dry tensile of throughdried tissue and towels can be increased by treating the pulp with a debonder, a wet strength agent and a dry strength agent. Chemical debonders, also referred to as softeners, are frequently employed in the manufacture of paper tissue and towel. One preferred debonder composition includes a softener system comprising a substantially equimolar, ion-paired mixture of an anionic surfactant and a cationic quaternary ammonium compound. Details are seen in U.S. Pat. No. 6,245,197 to Oriaran et al. Typically, debonders are added to the papermaking furnish at relatively low fiber consistencies, such as are seen in a stock chest or a machine chest. In this regard, see U.S. Pat. No. 5,785,813 to Smith et al.; note FIG. 1 thereof. Note also, U.S. Pat. No. 5,501,768 to Hermans et al., Example 9, Col. 13 wherein kraft hardwood fiber is treated with debonder in a shaft disperser.
The following patents also disclose papermaking processes wherein a debonder composition is added after the fiber has been pulped: U.S. Pat. No. 6,273,995 to Ikeda et al.; U.S. Pat. No. 6,146,494 to Seger et al.; and U.S. Pat. No. 4,441,962 to Osborn, III.
It has been suggested to pre-treat high yield fiber with a combination of oil and surfactant, prior to making absorbent sheet. In this regard reference is made to U.S. Pat. No. 6,001,218 to Hsu et al. and U.S. Pat. No. 6,074,527, also to Hsu et al. According to the '218 and '527 patents, a pulp slurry is treated at elevated temperature with oil and surfactant in order to produce softer products.
It will be appreciated by one of skill in the art that the prior art is replete with pulp treatments seeking to provide a softer and/or stronger product. In this regard, the following references are noted generally: United States Patent Publication No. 2003/0024669 (U.S. Ser. No. 09/852,997) entitled “Use of Hydrophobically Modified Polyaminamides With Polyethylene Glycol Esters in Paper Products” of Kokko; United States Patent Publication No. 2002/0162635 (U.S. Ser. No. 10/143,674) entitled “Softer and Higher Strength Paper Products and Methods of Making Such Products” of Hsu; United States Patent Publication No. 2002/0088575 (U.S. Ser. No. 09/942,468) entitled “Enzymatic Treatment of Pulp to Increase Strength” of Lonsky et al.; United States Patent Publication No. 2004/0123962 (U.S. Ser. No. 10/335,133) entitled “Amino-Functionalized Pulp Fibers” of Shannon et al.; U.S. Pat. No. 6,582,560 entitled “Method for Using Water Insoluble Chemical Additives with Pulp and Products Made By Said Method” to Runge et al. See also United States Patent Publication No. 2003/0159786 (U.S. Ser. No. 10/389,073) entitled “Method For Using Water Insoluble Chemical Additives with Pulp and Products Made by Said Method” of Runge et al.; United States Patent Publication No. 2004/0045687 (U.S. Ser. No. 10/242,571) entitled “Method for Using Water Insoluble Chemical Additives With Pulp and Products Made by Said Method” of Shannon et al.; U.S. Pat. No. 6,344,109 entitled “Softened Comminution Pulp” to Gross; and United States Patent Publication No. 2002/0074097 (U.S. Ser. No. 10/017,361) entitled “Softened Comminution Pulp”, also to Gross.
It has been found in accordance with the present invention that debonder pre-treatment of pulp further enhances sheet properties of regenerated cellulose microfiber containing products.